The clinical study of thrombocytes is rapidly gaining recognition not only in diseases connected with the hematopoietic system but also in infectious and metabolic disorders. Stahl1 and others have recently pointed out the importance that the number, the size and the staining of the thrombocytes play in the diagnosis and prognosis of various diseases. This discussion will be confined chiefly to the rôle played by the thrombocytes in hemorrhagic diathesis.

For a long time the thrombocytes had been considered as imperfect or deformed red cells, that is, the débris of red blood cells. In 1877 Hayem described thrombocytes under the name of hematoblasts because he believed these structures to be youthful forms of red blood cells. In 1881 Brohm observed two cases of Werlhoff's disease and was the first to point out that in this affection the hematoblasts were markedly diminished.